There's a nice free rose here:

http://www.robinwood.com/Catalog/FreeStuff/FreeModels/FModelPages/PoserRose.html

If the petals look black, invert the normals.

And there are lots of nice flowers, including roses, here:

http://toucan.web.infoseek.co.jp/3DCG/3ds/FlowerModelsE.html

You could make your own bouquet.